1. Technical Field
The present technical field relates to a ladder-type elastic wave filter and an antenna duplexer using the same.
2. Background Art
A conventional ladder-type elastic wave filter is described with reference to a drawing. FIG. 13 is a circuit block diagram of a conventional antenna duplexer which includes a ladder-type elastic wave filter.
In FIG. 13, conventional antenna duplexer 101 is, for example, an antenna duplexer of the UMTS system, and includes ladder-type elastic wave filter 102 as a transmission filter and reception filter 103 having a passband higher than a passband of ladder-type elastic wave filter 102. Ladder-type elastic wave filter 102 is connected between input terminal 104 and antenna terminal 105, receives a transmission signal from input terminal 104, and outputs it from antenna terminal 105. Ladder-type elastic wave filter 102 includes series resonators 108, 109, 110, and 111, and parallel resonators 112, 113, and 114 having a lower resonance frequency than antiresonance frequencies of the series resonators, which are connected to each other in a ladder form. Furthermore, ground terminal 117 sides of parallel resonators 112, 113, and 114 are connected to each other at connection part 116, and ladder-type elastic wave filter 102 includes inductor 115 connected between connection part 116 and ground terminal 117.
Furthermore, reception filter 103 includes, for example, resonator 119 and longitudinal mode coupled filter 118, which are connected between antenna terminal 105 and output terminal (balanced terminal) 106. Reception filter 103 receives a received signal from antenna terminal 105 and outputs it from output terminal 106.
Furthermore, antenna duplexer 101 includes phase shifter 107 connected between ladder-type elastic wave filter 102 and reception filter 103. Phase shifter 107 allows the passband of each of the transmission and reception filters to have a higher impedance in view of each other so as to improve isolation to each other.